The Atomic Force Microscope is a well-known device in which the topography of a sample is sensed by a tip mounted on the end of a microfabricated cantilever. As the sample is scanned, the interaction of forces between the tip and the sample surface causes pivotal deflection of the cantilever. The sample topography is determined by detecting this deflection. The AFM technology has also been applied to the field of data storage with a view to providing a new generation of high-density, high data-rate data storage devices for mass-memory applications. AFM-based data storage is described in detail in IEEE Transactions on Nanotechnology, Volume 1, number 1, March 2002, pages 39 to 55, Vettiger et al., and in IBM Journal of Research & Development, Volume 44, No. 3, May 2000, pp323-340, “The ‘Millipede’—More Than One Thousand Tips for Future AFM Data Storage”, Vettiger et al., and the references cited therein.
The cantilever-mounted tip, referred to also as the read/write tip, is used for reading and writing of data on the surface of a data storage medium. In operation, the read/write tip is biased against the surface of the data storage medium. The storage medium generally comprises a polymeric material.
In the write mode, the read/write tip is heated which results in heat transfer to the polymer surface layer causing local softening of the polymer. This allows the tip to penetrate the surface layer to form a pit, or bit indentation; such a pit represents a bit of value “1”, a bit of value “0” being represented by the absence of a pit. This technique is referred to as thermo-mechanical writing.
The storage medium can be moved relative to the read/write head allowing the tip to write data over an area of the surface, or “storage field”, corresponding to the field of movement. Each indentation is created by heating the cantilever tip and with the application of force pressing this tip into the polymer film. The tip is heated by passing a current through a resistive heating element integrated in the cantilever, directly behind the tip. Some of the heat generated in the resistor is conducted through the tip and into the polymer film, locally heating a small volume of the polymer. If sufficient heat is transferred to raise the temperature of the polymer above a certain temperature (which is dependent on the chosen polymer), the polymer softens and the tip sinks in, creating an indentation or bit.
In the read mode, the storage field is scanned by the tip, the position of the tip and the cantilever on which the tip is mounted differs according to the presence or absence of a pit. The reading operation uses thermo-mechanical sensing based on the principle that the thermal conductance between the cantilever, and components attached thereto, and the storage substrate, changes according to the distance between them; the distance is reduced as the tip moves into a bit indentation. Further discussion of the reading operation can be found in the above identified IBM Journal of Research & Development article.
In a multi-cantilever/tip device, multiple simultaneous operations can be carried out on a common polymer substrate by individually addressing each bit location. By virtue of the nanometer length-scale of each operation, this array of multiple bit locations in sum occupies a minimum amount of space constituting an ultrahigh density ‘reactor’. Data are stored by making nanoscopic indentations in a thin polymer film using a highly parallel array of cantilevers. As described above, at each position, an indentation or pit represents a 1 and no indentation or pit represents a 0, therefore data can be stored in a traditional binary sense via the presence or absence of nanoscopic indentations in the thin polymer film which serves as the storage medium.
In such a data storage system, a single tip of the tip array has to be able to read and write up to hundred billion indentations during the system lifetime. However, due to such a mechanical exposure, wear at the tip can be observed during the system lifetime. If the tip gets blunt over time, the error rate increases until the device fails completely.
It is thus desirable to support a data storage system with low error rates due to an appropriate handling of tip-wear.